In the age of introduction of digital film processing and digital cinema, multiple digital processing steps appear in the—classically analogue—cinematographic workflow. While digital special effect generation and post-production have been common for some time, digital cameras and new digital display devices and digital media now begin to be widespread. The digital equipment completely changes working habits during capture and post-processing and while digital technologies bring new features, they also represent a danger for artistic experience and heritage. It is thus necessary to transfer known artistic effects from conventional—analogue—cinematographic post-processing into digital cinema post-processing. Since the introduction of new equipment is expected to be gradual, technologies for co-existing analogue and digital processes have to be studied. This may for example concern a film captured by an analogue film camera and then displayed by a digital projector, or integration of digital special effects in an analogue film production chain.
One of the artistic workflow tools is colour correction, which for example is applied to raw film material after production to compensate for illumination colour artefacts or, before film distribution, to fine-tune the colour tones in order to realize artistic intents for certain scenes. Apart from being an important step of cinematographic post-processing, colour correction is also applied to photographs, paintings or graphics before printing.
Colour correction can be applied to a sequence of video frames, to a single video frame, to still images or even to parts of an image, such as an object therein. It is usually performed in cooperation between an artistic director and one or more skilled operators. The artistic director describes the intent of the colour correction while the operator transforms the intent into a colour transform applied to the visual content. Such colour transforms may for example include an increase of saturation, a change of colour hue, a decrease of red tones or an increase of contrast. Colour correction may be global to an image, to a set of images, to a specific region in one single image or even to all image regions in several images corresponding to a specific semantic unit.
During colour correction, the artistic director and the operator have to keep in mind what the impact of the applied colour correction will be on the final reproduction medium. The following examples illustrate this problem. In a first example, a painting is scanned and colour corrected using a personal computer (PC). The operator verifies the applied colour correction on the display of the PC, but the final reproduction is done on paper printer. A second example is a film that is scanned, digitalized, and colour corrected using a dedicated high-resolution colour correction device. The operator verifies the applied colour correction on the screen of a high definition control monitor, but the final reproduction is done by a film printer followed by film projection.
In both of these examples, the verified colour may be different from the reproduced colour, any differences between the proof viewing display device (for example the PC monitor screen or the high definition control monitor) and the final reproduction device (for example a paper printer or a film printer followed by film projection) should be taken into account during colour correction. These differences can include changes of hue, changes of saturation, changes of contrast, changes of luminance, changes of dynamic range, changes of colour gamut.
A known, partial solution to this problem is colour management (CMM). For CMM, the characteristics of the proof viewing device and the final reproduction device are measured, mathematically modelled, and then compensated for, using a colour transformation. CMM takes into account the colour gamut, which describes the totality of reproducible colours of a display device, of the devices involved. When an image contains colours outside of the gamut of a display device or close to the border of the gamut, the applied colour transform may contain colour compression, colour clipping or other specific operations such that the transformed colours are inside of the gamut.
It is easily realised that the difference between colour gamuts of display devices causes a problem for colour correction. It may happen that the operator applies a colour correction that generates an acceptable result on the proof-viewing device while the final reproduction device is not capable of reproducing some of the colours, since the colour gamut of the final reproduction device is different from that of the proof-viewing device. It may also happen that the operator wants to apply a specific colour correction which would generate acceptable results on the final reproduction device, while the correction cannot be sufficiently visualized on a proof-viewing device with limited colour gamut.
A second problem of colour correction is caused by the colour transformations of CMM. These colour transforms transform input colour values to output colour values. The transforms may comprise several partial colour transforms, each defined for a specific range of valid input colours. On the borders between such ranges of valid input colour, the colour transform may change its slope or be discontinuous. This may generate false contours in the transformed image. What's more, colour transforms often use Look-Up Tables (LUTs) that contain a set of pairs of sample input and sample output colours. When colour is transformed in a LUT, colours in-between the sample colours must be interpolated. Colour interpolation may introduce discontinuous colours or discontinuous slopes of colours at sample colours, which may generate false contours in the transformed image.
It is well-known in the art to detect colour values outside a colour gamut of a reproduction device, so-called “out of gamma alarm.” For example, US patent application US 2003/0016230 A1 teaches a system that indicates all colours from a colour palette that are outside the gamut of a reproduction device.
The present invention intends to improve upon the prior art by providing a way for an operator to be informed of possible problems in a reproduced image caused by colour correction.